The present invention relates to a method and device for converting the bit rate of coded voice data and, more particularly, to a method and apparatus for converting the bit rate of coded audio data which are used with a device for recording, transmitting or reproducing digital speech.
Prior art examples of high efficiency audio coding techniques are standard audio coding systems proposed by JTC (Joint Technical Committee) of ISO (International Organization for Standardization) and IEC (International Electrotechnical Commission), that is, MPEG1 Audio (Moving Picture Expert Group) and MPEG2 Audio. Coded by the MPEG Audio system, the voice or speech sound signal is converted to data of a transmission rate substantially in the range of 32 to 448 kbit/s according to its audio quality, and the coded data is stored in a computer hard disk or transmitted via LAN (Local Area Network).
On the other hand, transmission lines of the Internet and ISDN can be used at various transmission rates according to each line traffic or each transmission speed contracted; hence, there is also a demand for audio data transmission at various transmission rates. To meet with this demand, a technique is required, for example, to decode once-stored coded data to reproduce original audio signal and encode it again in accordance with the required transmission rate.
FIG. 6 shows an example which employs an ordinary coder and an ordinary decoder to satisfy the above-mentioned demand. As shown in FIG. 6, data coded at a coding bit rate R1 and input via a coded data input terminal 11, is provided as a speech signal and as coded information at outputs 18 and 19 of a decoding section 13, respectively. These outputs and a target coding bit rate R2 via an input terminal 12 are input into a coding section 1A, from which coded data newly converted to the coding bit rate R2 is provided at a coded data output terminal 1K.
At present there is no established technique of directly converting coded audio data to a required coding bit rate; hence, it is conventional to use a method for reconstructing an audio signal from coded audio data and encoding it again at the required coding bit rate. For example, in the MPEG audio, however, very high level processing is required for fast processing of convolutions by a subband analysis filter band which make up a majority of the computational load involved in coding and decoding processes. The conventional method has another disadvantage such that since a psycho-acoustic analysis is carried out again on audio data decoded from coded data, the audio quality is likely to be degraded by targeting of deteriorated coded audio data different from an original audio data.